A hot dip galvanized steel sheet is generally utilized as a steel sheet for automobiles, a material for civil engineering and architecture, or a heat resistant or corrosion resistant material for home electric appliances, and so on. Most of such hot dip galvanized steel sheets are produced mainly by continuous molten zinc galvanizing treatment. Apparatuses used for such continuous molten galvanizing treatment are provided with a dip roll immersed in the molten zinc, rolls disposed near the molten zinc surface, guide rolls for guiding the galvanized steel sheet after passing these rolls, etc. In addition, an injection nozzle, which blows a high pressure nitrogen gas along the steel sheet passing above the galvanizing bath, is equipped on the apparatus in order to control the amount of molten zinc which adheres to the steel sheet which is pulled up from the molten zinc.
These rolls and the injection nozzle are immersed in the molten zinc, are subjected to adherence of the scattered molten zinc, or are contacted with a highly-heated steel sheet coated with molten zinc. Especially, a sink roll for the zinc galvanizing bath, which is one of the major equipments for the hot dip galvanized steel sheet, has a problem that the sink roll degrades due to corrosion by reacting with the molten zinc and to adherence of intermetal compounds and the degraded part adversely affects the quality of the zinc galvanized steel sheet as a product.
In order to address this problem, Japanese Patent Laid-Open Publication No. H9-25583 proposes a ceramics-coated roll for molten zinc bath, in which the surface of the roll has been thermally sprayed with a carbide containing 85% by weight or more of WC, followed by surface coating with a ceramic film comprising at least one of the nitrides or carbonitrides of Ti, Al, Hf, or Zr.
In addition, Japanese Patent Laid-Open Publication No. 2004-331995 proposes a surface coating material for an immersion member of a molten metal galvanizing bath, in which 50 to 90% by mass of a ceramics powder comprising one or more of oxide, nitride, carbide, boride, or silicide is dispersed in a powder of one, or an alloy powder of two or more, selected from Ta, Ti, V, Mo, Cr, Zr, Nb, Hf, or W, which is a high melting-point metal powder having a melting point higher than that of Co.
The surfaces of the rolls proposed in the aforementioned Japanese Patent Laid-Open Publication No. H9-25583 and No. 2004-331995, generally referred to as a sink roll, is treated with thermal spraying with a cermet comprising tungsten carbide (hereinafter referred to as “WC”) and Co (hereinafter referred to as “WC/Co cermet”), in order to improve the zinc corrosion resistance while retaining the abrasion resistance. However, the zinc corrosion resistance is not sufficient and the degradation of the roll surface may cause the defect of the product. Especially in recent years, due to the strict quality requirements for the steel sheets for automobiles, even the materials disclosed in Japanese Patent Laid-Open Publication No. H9-25583 or No. 2004-331995 and WC/Co cermet thermal sprayed materials require maintenance at least about every month. It is therefore the current situation that further improvement in zinc corrosion resistance is required. In addition, Japanese Patent Laid-Open Publication No. H9-25583 has a problem that cost and processing time are increased for performing two types of thermal sprayings. Japanese Patent Laid-Open Publication No. 2004-331995 has a problem that the thermal spraying process condition is restricted for melting and scattering the high melting-point metal.